Lobopodia
Encyclopedia
- For a type of cell projection see PseudopodPseudopodPseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
.
Lobopodia is a group of poorly understood animals, which mostly fall as a stem group of arthropods. Their fossil range dates back to the Early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
. Lobopods are segmented
Segmentation
Segmentation may mean:*Market segmentation, in economics and marketingBiology*A process of morphogenesis that divides a metazoan body into a series of semi-repetitive segments*Segmentation , a series of semi-repetitive segments...
and typically bear legs with hooked claws on their ends.
The oldest near-complete fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
lobopods date to the Lower Cambrian; some are also known from a Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
Lagerstätte. They resemble the modern onychophorans (velvet worms) in their worm-like body shape and numerous stub-legs. They differ in their possession of numerous dorsal
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
armour plates, "sclerita", which often cover the entire body and head. Since they taper off into long, pointed spikes, these probably served a role in defence against predators. Individual sclerita are found among the so-called "small shelly fauna
Small shelly fauna
The small shelly fauna or small shelly fossils, abbreviated to SSF, are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian period. They are very diverse, and there is no formal definition of...
" (SSF) from the early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period. The "lobopodia" group is considered to include these Cambrian forms in addition to the onychophorans.
Representative taxa
The better-known generaGenera
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
include, for example, Aysheaia
Aysheaia
Aysheaia was a genus of soft-bodied, caterpillar-shaped organisms average body length of 1–6 cm. The genus name commemorates a mountain peak named "Ayesha" due north of the Wapta Glacier. This peak was originally named Aysha in the 1904 maps of the region, and was re-named Ayesha after the heroine...
, which was discovered among the Canadian Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
and which is the most similar of the Lobopoda in appearance to the modern velvet worms; a pair of appendages on the head have been considered precursors of today's antennae. Xenusion
Xenusion
Xenusion is a possible very primitive arthropod/onychophore known from two specimens found in glacial debris in Germany. They probably originated in the Kalmarsund Sandstone of Southern Sweden and are probably late Neoproterozoic. The specimens are not especially well preserved...
was apparently able to roll itself up, spines outward, giving insight into the defensive strategies of the Lobopoda. However, by far the most famous of the lobopod genera is Hallucigenia
Hallucigenia
Hallucigenia is an extinct genus of animal found as fossils in the Middle Cambrian-aged Burgess Shale formation of British Columbia, Canada, represented by the species H. sparsa, and in the Lower Cambrian Maotianshan shale of China, represented by the species H. fortis...
, named on account of its bizarre appearance. It was originally reconstructed with long, stilt-like legs and mysterious fleshy dorsal protuberances, and was long considered a prime example of the way in which nature experimented with the most diverse and bizarre body designs during the Cambrian. However, further discoveries showed that this reconstruction had placed the animal upside-down: interpreting the "stilts" as dorsal spines made it clear that the fleshy "dorsal" protuberances were actually legs. This second reconstruction also exchanged the front and rear ends of the creature, which further investigation showed to be erroneous. The armoured lobopodian Diania
Diania
Diania is an extinct genus of animal found in the Lower Cambrian Maotianshan shale of China, represented by a single species D. cactiformis. Known during its investigation by the nickname "walking cactus", this remarkable organism belongs to a group known as the armoured lobopodians and has a...
is significant for having the most arthropod-like appendages.
Morphology
Most lobopods are in the order of inches in length. They are annulated, although the annulation may be difficult to discern on account of their close spacing (~0.2mm) and low relief.Lobopodia and their legs are circular in cross-section. Their legs, technically called lobopods, are loosely conical in shape, tapering from the body to their clawed tips. The longest and most robust legs are at the middle of the trunk, with those nearer the head and tail more spindly. The claws are slightly curved. Their length is loosely proportional to the length of the leg to which they are attached. The eyes are similar to those of modern arthropods as has been shown in Miraluolishania
Miraluolishania
Miraluolishania is a lobopod known from the Chengjiang biota. It is remarkable for the possession of lensed pit-eyes....
haikouensis (Liu et al., 2004).
Their gut is a straight, undifferentiated tube, which in some fossils is found to be filled with sediment. The gut of the representative Paucipodia
Paucipodia
Paucipodia inermis is a lobopod known from the Lower Cambrian Chengjiang lagerstätte. Its gut is puzzling; in some places, it is preserved in three dimensions, infilled with sediment; whereas in others it may be flat...
is variable in width, being widest towards the centre of the body. Its position in the body cavity is only loosely fixed, and flexibility is possible.
Ecology
The way of life of the Cambrian Lobopodia is to a large extent unknown; some species were apparently carnivorous, feeding on other animals such as, for example, sponges (Porifera). Others apparently lived in close association with the enigmatic genus EldoniaEldonia
Eldonia is a soft-bodied animal of unknown affinity, best known from the Fossil Ridge outcrops of the Burgess Shale, particularly in the 'Great Eldonia layer' in the Walcott Quarry...
.
Diversity
During the Cambrian lobopods displayed a substantial degree of biodiversityBiodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
. However, only one species is known from each of the Ordovician and Silurian periods, with a few more known from the Carboniferous (Mazon Creek).
Phylogeny
The lobopods are closely related to the arthropods; indeed, the arthropods may well have arisen from within the lobopods. They are less closely related to the tardigrades; precise classification is still in flux.See also
The name Lobopodia is also used for lobose pseudopodPseudopod
Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...
s, found among certain amoeboid
Amoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
s.